A mathematical model of direct sun and solar drying of some fermented dairy products (Kishk) was developed. Solar radiation, heat convection, heat gained or lost from the dryer bin wall and the latent heat of moisture evaporation were the main components of the equations describing the drying system. The model was able to predict the drying temperatures at a wide range of relative humidity values. It has also the capability to predict the moisture loss from the product at wide ranges of RH values, temperatures and air velocities. The model showed a dramatic effect of the air velocity on increasing the moisture loss at the beginning of the
drying process and became constant which leads to a recommend that it is better to use low temperature with forcing air at the beginning of drying. In the next stages, high temperature without forcing air should be used. The model was validated by using experimental data of the drying temperatures and moisture loss under both direct sun and solar drying systems for five fermented dairy products (Kishk). The predicted values were in a reasonable agreement with the experimental data. The coefficients of determination for the predicted moisture loss in case of the sun drying system were lower than those under the solar drying. This may be attributed to the deeper grooves and cracks on the surface of the products, the heat conducted from the drying trays and the variation of the measured wind speed. |
